1. Field of the Invention
Embodiments of the present invention generally relate to photovoltaic/solar cell and solar panel manufacturing.
2. Description of the Related Art
Photovoltaics (PV) or solar cells are material junction devices which convert sunlight into direct current (DC) electrical power. A junction develops a photo-voltage while the area and other parameters of the device determine the available current. Practical solar panels are sized in area to deliver the needed amount of power and optimize other application parameters. Solar panels are created by tiling a number of solar cells together whose dimensions are optimized by a variety of manufacturing constraints to minimize the manufacturing cost. The number of individual solar cells used is also determined by trading off between panel operating voltage and current (I), since the wiring generates resistance (R) and thus contributes a power loss (PL) given by PL=I2·R.
One example of PV systems includes a stand-alone system which in general powers for direct use or with local storage. Another type of PV system is connected to conventional utility grid with the appropriate power conversion equipment to produce alternating current (AC) compatible with any conventional utility grid. Overall, PV systems can generate power for many uses, such as remote terrestrial applications, battery charging for navigational aids, telecommunication equipments, and consumer electronic devices, such as calculators, watches, radios, etc.
A typical PV cell includes a p-type silicon wafer or sheet typically less than about 0.3 mm thick with a thin layer of n-type silicon on top, forming a p-n junction, which creates an electric field. When exposed to sunlight (consisting of energy from photons), the p-n junction of the PV cell generates pairs of free electrons and holes. The electric field of the p-n junction separates the free electrons and holes, creating a voltage. A circuit from n-side to p-side allows the flow of electrons when the PV cell is connected to an electrical load. Electrical power is the product of the voltage times the current generated as the electrons and holes recombine. Optimized solar cells usually mean maximum power generated at minimum cost.
Depending on the end use application, a variety of other parameters are considered. As an example, one or both surfaces of a PV cell are coated with suitable dielectrics after the p-n Junction is formed. The dielectric layers are used to minimize surface recombination and on the front provide antireflective coating to reduce reflection losses of photons. The bottom of the PV cell is generally covered with a back metal which provides contact for good conduction as well as high reflectivity. The front or sun facing side of the PV cell is covered with area minimized metallic contact grid for transporting current and minimizing current losses due to resistance through the silicon layers. Some blockage of sunlight or photons by the contact grid is unavoidable but can be minimized. Metal grids with patterns of conductive metal lines are used to collect current. The final step is a glass cover plate which protects the PV cell and provides structural re-enforcement.
Solar cells and PV panels currently are manufactured by starting with many small silicon sheets or wafers as material units and processed into individual photovoltaic cells before they are assembled into PV module and solar panel according to a variety of contacting or wiring schemes. These silicon sheets are generally saw-cut p-type boron doped silicon sheets, precut to the sizes and dimensions that will be used, e.g., 10 cm×10 cm, or 21 cm×21 cm. Originally these sizes were derived from available single crystal wafer sizes, but the advent of cast multi-crystalline silicon and ribbon technology has created more freedom in choosing sheet sizes for cell formation and optimizing production.
The cutting (sawing) or ribbon formation operation leaves damage to the surfaces of the precut silicon sheets, and one or more etching processes are performed on both surfaces of the silicon sheets to etch off about ten to twenty microns in thickness from each surface and provide surface textures thereon. Next, n-type doped silicon is formed by thermal or rapid thermal diffusion, e.g., by phosphorus diffusion penetrated up to a maximum of about 0.5 microns deep. Then, p-n junction isolation is performed at the edges of the silicon sheets by sand blasting, plasma etch or laser cutting. Additionally, the front side of the silicon substrates is covered with a deposited layer of antireflective coating made of a dielectric material, such as silicon dioxide, titanium dioxide, or silicon nitride which also provide as a barrier to surface recombination. Silicon nitride is currently the preferred primary dielectric film.
In general, screen printing thick-film technology is then used in the PV cell industry to layer a conductive paste of metal materials, e.g., silver, etc., into a desired pattern and deposit a metal material layer to the surface of the silicon sheets/substrates for forming metal contact fingers or wiring channels. Other thin film technologies may be used for contact formation or electrode processing. The deposited metal layer, formed into contacts, is often fired or sintered at high temperature to form into good conductors in direct contact with underlying silicon materials, and a single PV cell is made.
A solar module is formed by tiling a number of individual PV cells arrayed and bonded onto protective films and a carrier glass to create a solar panel appropriately sized for the required power output and wired to achieve the desired operating voltage and current. Currently, many different wiring/interconnect schemes can be used for contact patterning and current collection; for example, schemes using both front and back side wiring, schemes using front side current collection but all the contacts are brought to the back side, and other wiring schemes. Front side generally refers to the side facing sunlight.
FIGS. 1A-1E demonstrates a basic PV module fabrication process for PV cells. In general, as shown in FIG. 1A, after a single PV cell 110 is finished, metal tabs 104 are soldered to bus bars 102 on the surface of the PV cell. As shown in FIG. 1B, two or more metal tabs 104 per cell can be used to wire metal contacts or metal fingers 108 on each PV cell 110, provide interconnect links between PV cells 110, and allow thermal expansion. Regardless of size, a single PV cell generally produces about 0.5-0.6 volt DC current.
In FIG. 1C, several PV cells 110 are interconnected in series or parallel electrical circuits into a PV module to produce higher voltage, current, and power levels. Interconnection wiring of each PV cell 110 into strings 106 or modules 120 is performed by soldering and wiring metal tabs 104 and auxiliary tabs together, using various wiring schemes. A common configuration uses about 36 connected PV cells for a maximum of about 15 volts, compatible with major appliances and appropriate for 12 volts battery charging.
As shown in FIGS. 1D and 1E, the PV module 120 is usually stacked with encapsulant materials, such as ethylene vinyl acetate (EVA) sheets 122, and covered with a front glass pane 140 and a back pane 130. The PV module 120 can further be sealed in protective laminates or barriers. A number of PV modules 120 can also be assembled into pre-wired panels 150 or arrays 160. Protection of the active PV devices during module construction directly affects the performance and lifetime of the final PV systems.
PV module or solar panel is often completed by manual assembly, wiring, and soldering. Machines do exist for tabbing, laminating, and curing, but automation is in need of further improvement. Overall, the manufacturing process leads to relatively inefficient manufacturing volume for handling large unit wiring assembly. Currently, the whole manufacturing process for making an installed PV system is very expensive.
Therefore, there is a need to improve the scalability of the manufacturing process and find various ways to reduce the manufacturing cost.